Petechiae in 19 month old, please help!

I took my daughter to her check up back a month ago. I showed her doctor the dots that had appeared under her skin on her butt. She assured me it was Petechiae since she had just gotten over 2 back to back "colds" both with a high fever. At that point she had them for a few weeks. Now she has had them for about 2-3 months! Doctor said they would eventually go away, but some days they are barely there and now today there are probably 75 dots, mostly pin point, but a couple a little larger. Also there are 2 TINY dots on her hand and one little dot on the tip of her nose. If anyone has dealt with this please respond! Additional info, 19 month female, playing good, sleeping tons, not lethargic, does seem to be eating less- but was told that is common at her age.

I want to know:
1- how long does it take for these to go away.
2- is it common for them to spread-even 1 month after a viral infection?
3- why are they barely there one day and then more noticable next..

(this morning they were WOW there, and now tonight they are lighter and alot less of them.) I am a nervous wreck, and its bringing tears to my eyes thinking that this could be a sign of leukemia, yet my doctor has no concerns since she had just got over a cold. THANKS!